Reduction of cross talk



Sept. 14: V J T DlXON REDUCTION OF CROSS-TALK Filed April 26, 1941 2 Sheets-Sheet 1 FIG.

BALANCE UNI r BALANCE UNI T BALANCE UNIT MOD SUPERGROURf-Fy TO F8 v if}? INVENTOR F7 J 7. DIXON ATTORNEK Sept. 14, 1943.

J. T. DIXON REDUCTION OF CROSS-TALK Filed'April 26, 1941 2 Sheets-Sheet 2 SQQ - INVENTOR J. 7TD/XON mm 3 g m I. 8\ m NM. R. k z m 2 3 a. :6 m a kauvisow quzzxtu 3E A T TOP/VF V I PatentedSept. 14, 19 43:

REDUCTION oFcno ss'rAL aster. ixon, Newrork, my flassig nor to Bell Telephone Laboratories; Incorporated, ,New f York, n; corporation oi New York;

Applicationnpril 26, 19 4i;'seriai Noieaosoa I '1 claims. 01.179515) 'Ihisdnvention relates to systemsi orthetransmission of sig nals occupying: a wide frequency range and more particularly to" the reduction or neutralization of inter-circuit cross-talk in such Systems, I, a i v j v I i ,It is well known that-where signalling lines parallel each other. for :vany considerable dis: tance,;the signaling current in each line tends to induce disturbing cross-talk currents in the others. To reduce the disturbing efiect it is the cally areceivingterminaliin a'multiplex'carrier practiceto couple each pair oi lines with across- 1 talk balancing unit which ;is'; desfigned and. ad-

justed to introduce cross talk currents cit-such.

phase-and magnitude as toneutralize or; at least substantially reduce the cross-talk :thatis due to much as the amplitudelandl phase o'fthe coupling' between the lines varyflwith frequency, the I balancing unit m-ust have a coupling character istic that is likewisenon-uniform with respect undesired couplings. betweenflthe lines. Inas;

to ;fre'quency= and this characteristic must be maintained over the entirerange of. coupling values provided by the It will .be apprecig inaccordance with "one feature, cross-talk bale ancing is appliedto.bands of'signals derivedby one orfmore stages' otmodulationidrgdemodular tion "at points inflthe" system wherej such modu-j lation or' demodulation.is-(required; for signal routingxpurposes. f I f f I .Q 1-. g Inv accordance with another ,.feature the line frequency range" is successivelyv subdivided into component bands, by. demodulation, and cross-' talk balancing. is applied tola iplurality of the successive subdivisions..' Q The name, of the pr sent. nvent and i various features, objects and advantages will appear more fully from a "consideration of the several embodimentsillustrated inf'the acc mpanying'drawings'and now to be'described f- In the drawings: I h f.

Figs. 1, 2 and 3illustrateembodiments of the invention in which cross-tails" lancing -i I plied .to' demodulated signal components; and Fig, .4 illustrates an I embodiment J of the, inven-I5 tion in which cross-talk balancing is applied-to. subdivisions? of the line frequency rangewith: outl frequency translation, i

Fig; l'is av block diagram showing schemati-- telephonetsystem adapted to receive and separate the various message channels. incoming .over transmissionllines I andZ. It maybeassumed thatithetwb lines comprise separate circuits in. a multipair cable .or that they are otherwise-in cross-talk producing relation. .1-

1,: 5: Itliwilltb'e assumedfor specific example that eachof the linesl' and 2 carries amultiplex of carrier telephone'channels rangingin frequency from 64"t'o'2064 kilocycles per second. Inthe re:; ceiving. process the message currents taken-from each line are first separated by filters l2 intoa multiplicity of sub-bands and these are applied to respective demodulating. equipments It by means; of. which all of thesub-bands are trans;

lated to the same frequency range. -'-Ihus the ,two-millioncycleband of line frequenciesmay be divided into eight sub-bands or supergroups ofxa'bout 250 kilocycles each-, each supergroup,

thenbeingtranslatedto thefrequency range 312 to i5 52;kilo'cyc1es, dueallowance being made forspace betweenchannels; Each-demodulated supergroup :is then divided :into its component groups by filters 4 and theseare translated by individualidemodulating equipments 5 toa com mon group. frequency range. Thusthere may. be :five groups'each" having a width of about 4.3 kilocycles, and-the common-frequency range may.

lie betweenand 108 kilocycles. -By a'further stage. of demodulation (not illustrated) the groups .may, in turn, be separated into. their componentchannels and the latter-reduced to voice frequency. v I 1 v i--Whatever the particular manner of subdivi sion* in the successive stages of the demodula-Q tion may be, it is the same for both of the lines I and .2. .With this identity of treatment, it rellows that any pair of supergroups that occupied the same "frequency position on the two-lines .and which therefore were in. cross talk relation, will b'e broughtjvout fromtherespective demodulati ing equipments l3 with unchanged, total, band width and occupying identical positions in the irequency"spectruxn. {Similarly any. pairof cross-talking groups on the respective transmission linesl and 2' areeventually isolated from all Jotherg'roups and brought out from demodulators 5 at theisame'frequencylevel. Likewise,.

of course, the individual channels are eventual tion adapted for a system of the kinddescribed ly isolated and brought out at voice frequencies with reference to Fig.1 and provided with cross- O efi ct t cross-talkbalancing b en 6 talk balancing equipment in accordance with the p group and another, a ba an unit, c invention; At the repeater station shown it may as 0r S Co l d between the tputs o t e 5 be supposed that the cross-talk on the line in re p t e y Correspbnding demodulators l3. certain supergroupshas become undesirably ach ofrthe ba a ci un etc-l sh quired large and thatreduction of. the cross-talk in these to be effective only oyer the commqn$uperroup frequency-ranges i d i d; f frequency range of 312 to 552 kilocycles. The As illustrated, it is further supposed that one qu n y hara t st s of t s a an ine 0 supergroup is to be diverted from each of the units, however, are not q e alike o t S O C um lines for transfer to a branch line. Assumbe appreciated that whereas the balancing isie ih for specific example that supergroup No. 2 is fected at the Sa e Supergrollp qu n lfiyel, theone that. is to cbe transferred to a branch line the cross-talk to be neutralized arises'atdifier th t p r N 4 and No, '7 are the ones ent line frequencies involving difierent -cross-talk igl it y gg v b 4 1k, th th supergroups f q y Characteristics That Q p i are barred from the repeater amplifier in each ba a ci u 5 y introdllGeacol-lpliing which line-by-a band elimination filter 2i and they are increases sharply With frequency if at line free sgpawteyy diverted from th li by ppr iat quencies the supergroups it connects are subject" band-passing supergroup filters 22 A11 thregare to a sharply inc e s crossetalkfrequmcr then passed through demodulating' equipments p the Other at the E116 23 which may be the same asthe demodulators q c G'QuP- y another P Of P l3 of Fig. 1& ThedemodulatedsupergroupNa 2 p vth' Cross-m1]!- fireqllemly @hfliracteristic is then brou'ght'out fofapplicationto modulatmay be comparatively flat respect" ing equipment associated with the branch line, fluen y, in which case thercarres-ponding cross 25 and a supergr'oup: crosstalk balancing unit'24 ta k balancing u t T isvld'esighfidltoi have a P is connected between the outputs of the correrespondi n ly' t i -f n y har s endin pairof demodulating equipment- 23. acteri'sti'cfor all setti 1 The other two" demodulated s'upergroup's are re- Whether or not a y corresponding PW 0f stored to their lifn frequencies by means of resupergrou-ps. are connected in cross-talk balanc- V ti'v modulators flan-d applied td the o t. im; relation, one or more of. thepairs'of'group i H sIfl -du h1111 6 123; I components th r of may be coupled by Balancing unir25 interconnects the pair" of u 3, at Outputs of the respec' l, demodulated superg roups N0: 4- and balancing tiveiy corresponding demodulator equipments 5:. n g zs i simila ly Provided for'fihe P of N0; 7

Balancing units. 8 9, etc, are allialilre with re- 5 supergroups Assmtheicase frli 1, thesupelzp spect to theirequencyrange for winch they are gmupbammmg u a each: designed a adapted, vfz., 6U to= I- ofif'kil'ocycles, and; they differ limitedv frequencyg a has. a s; from each in that their respectiv? trans mission-frequency characteristic" appropriate to mission-frequency characterlsticsare designed; to the ne frequency ofitsi associated supergroups; match the cross-talkwliplingrfrequemr Chane 4 In 'the' embodiment of the invention, illustrated teristics obtain-meat the line? frequenc es of the v Fig 3 two r m a s a W'ar can? respectively corresponding w'ei, nected by apair of transmission [llhes 'land 2",

The arrangement describe? W Ieference: V eachofwhich'isarrangedforthe transmission of is afiexible'and' adapt" afplur ality'v ofcarrier telephone channels in the d'to cope with any'interci'rcuit' cross-talk'lchar .45. WEdirectibn T fiquenCyTr-ange: a for acteri'si'lic that may be encountered. With some the transmission of a mumpficity pfrghannelsfm ypes of'l'inesifor example, the crossetalkin al en: .v opposite .direQtipn another fiequgncy titularsupe ew fr q en rang? a be range. A far-endcross-italk problem iszp'resente'd parativel'y small or it may: be simple function inasmuch as each Set dfsignals r. line or'frequ and: in either @f these a e Fl cross-talks into the like-directed saof signals suifice to providefor'terminal cross-talk: balanc inv'the other. line; c i -f fi 5 nf in ween e PBJI'SG 1 v r v 's 'n' ecause 'an'cm um 5 other ine frequencyfirange 011 the 'handmhe 351:: the line' conductors tend "to int roduce crossk maybe'wmpamfively great'and'threj; afnear end cross-talk jcoupllngf'vvhich may de fore r q accurate 61105543111; balancing or m grade other circuits on the line. JFor example; 'may'beso noneuniform ith. respect tofreduency if a lvoicfrequeficy .-cn:cuit, with a l it are; tfiffgiti lifi is We b er e req e e k em' on ac o e airs gYsingle-balancing'unit. In either of these latfgr v6i) 21m ghg ggf g i i i m i cas s t s rg g ggg g gg'gzfigs 2; tween the pairsto reduce far-card croszatatlk'bes rue ionr erm1 a ranse gizx ii i atecross-taik balance, and the residual-or g lg i z gg ii gpliggg; gm' i Output uncompensatedcross-talk may be'counteracted of the repeater. on bail; th with th necessary d 6 0i accuracyhy means input of the E -W voice repeater" on pair 2" or of theinterg p mi units 8495 Obvi' vice versa thus producing near-end cross-talk 0115 35 these: wefl f pih i betweenithe Voice frequencysystemsr, crossdesired; balsa-.ncmg' re vome n v talk is, oicourse,,made more serious by the difl c Mme I ferences in level between the voice repeater out-s J l v i l m- I- V 1 Whereas'Fig 1 mush ates the mventmn as e puts and inputs; These difiicultiesare largely had in a receiving terminal station, the. irr- I l v sg tionhas application also to the-z reducti'oni 0i and other advantages securedby the 3 Fig 2 illustrates schematically a repeatef t line, the several carrier telephone channels from the respectivelyicorresponding"channe1 transmit ting equipment: are i'assembled in a single band ranging for example fr'om 6Q to 10$ kilo'cycles; This group is then passedithrough its'respective 'moiiu-- lator 3-H {amplified and appl'ied to sue" for 2 through alow-pass filter 32'. f'I'helin'e frequency range for this group may b'e 36 to 84-ki1ocycles. The east terminal is assumed tobe arr nged similarly to apply to each of the lines-"atband ofcarrier telephone: channel's having "a range off92 t6 '-1 40 kil'odyclfes. The receiving equipment at the west terminal comprises for each line a high-pass filter 33 and a demodulator 34 adaptedto translate the line frequency' band of 92 to 140 kilocycles to the '60 to 108 kilocycle range. The eastterminal issimilarly provided,

with low-pass filte'rs 31' and "demodulators 38 adapted to translate-the incomingpairs of136 to 84 kilocycle bands to the 60"to 108 lgil'ocycle range for application to the channel equipment.

For the reduction of 'cro ss talk a balancing unit line; frequency 35 is connected between the outputs" ofthe two receiving demodulators 34 at the 'west' terminal and a balancingun'it-SG' is summit connected between the outputs ;of the receiving demodula tors38 at the east terminal. Whereas the line frequency range involvedin the production of cross-talk is 36 to .140 lrilocycles, each of the balancing; units'need be adapted to cover only half of this range and the problem'ofsimultaneously balancing oppositely directedchannels for I crosstalkis'avoided; Each balancing unit is designed porating compensating phase shift in the modulator or demodulator equipments or. in the balancing units. c

Fig. 4 shows an embodiment of the present invention in which cross-talk balancing is effected 0n the lineand at line frequencies, the

latter being subdivided in the cross-talk balancing equipment into groups, without frequency transinone orthe iother ofthe'f-twofline frequency densers -44 and separated by a respective set of band-pass' filters 45, into their respective com-- ponent frequency bands. Each 'filter 45 .is' then connected through a balancing unit 41 tofa' filter 46 tha't covers the same line frequency range. An advantage with respect to -ease and accuracy of cross-talk balancing may be obtained by cor= relating in Fig; 1; Whereas eight pairsof filters" 45, '46-, each adapted to select one'of the supergroups on the line; might therefore be employedt'o advantage','l'only four such pairs are indicated Suit able correlation of the kind contemplated; however, may be obtained'by designing eachpair of these filters to pass-two adjoining'supergroups'; Alternatively, however, the bandwidths may'be unequal, and specifically, for example,-the width of eachband may be 'adjusted 'so that it is a fixed fraction of the mean band fr equency.= Alternaftively, too, balancing station '43 may coincide with a repeater station; in .which case the power separation filters canserve in "euof power blockin 1 m t 44,5 7 a k p' Whatever the particular manner l of line frequency subdivision may. be at the balancingsta tion 43,each' of the balancing unitsneedbead'apt: ed to cover only a traction of the' 'tot'al line 'fre quency range. "As inthe otherernbfodiments"de i I scrib'edhereinbefore, eachlofithe balancingunit's introduces. constant frequency weighting accord ingto the linefrequenciesfinvglvedin thebal ancing circuit.

:In a complete"transmissionsystem, a; balancing arrangement of the lsindshown in Fig. 4ma be utilized inzeachrepeater section, or in' occasional r'e'peater sections', to effect approximate cro'ss ,talki balancing in the P several-major subdivisions of the line" frequency range, and balancing units 41' of exceedingly simple construction may be em ployed for this purpose. At longer intervals the arrangement shown in Fig. 2 may be employed if certain major groups need special attention, and the balancing units 25, 26, may in this case have fairly precise coupling-frequency characteristics. 1 At the receiving terminal of the system, the ar- Y rangement shownin Fig. 1 may be employed to lation, and separately balanced from cross-talk. The system shown may be a part of that described with reference to Fig. 1, and it will be assumed for purposes of exposition that .it is and that the line frequencies involved are therefore the same, viz., 64 to 2064 kilocycles; The two transmission lines I and 2 are illustrated as comprising separate pairs which may form a quad under a common shield or sheath, for example. Two successive repeater stations 4| and 42 are shown and each comprises a pair of repeater amplifiers for W--E transmission through the two lines. Associated with each repeater amplifier is a pair of power separation filters so that direct current or low frequency power supplied over the respective pairs may be passed around each repeater amplifier for'through transmission and a part diverted for energizing the power supply unit of the amplifier. Similar arrangements are, of course, provided for E-W transmission at the repeater stations.

Midway between .the two repeater stations is a cross-talk balancing station 43. At this point the message currents are taken off each line through a respective set of power blocking conreduce the residual cross-talk to a tolerable value in all supergroups, groups, or channels where such additional balancing is required. 7

Although the present invention has been described with reference to several illustrative embodiments thereof, these embodiments are not m tended to be limitative, for the invention is susceptible of application in various other forms within the spirit and scope of the appended claims.

What is claimed is:

1.-In combination, a pairof longdistance transmission lines in cross-talking relation ,both carrying signals in the same wide carrier frequency range, means at a. signal receiving point commonto said lines for separating the signal frequencies on each line into a multiplicity of groups, means for translating the separated groups to the same frequency position, means for bring a plurality of the translated groups groups from the other line, means for further subdividing said last-mentioned translated groups into subgroups, means for translating said subgroups to a common frequency position, and" means for bringing. at least one of said transthe sub-division of the :line frequency range at balancing station43 withtlie-subdi'vision that is effected 'at the terminal station's as shown lated v subgroupsinte crosseta lkbalancing; rela' tiorr with-the corresponding. translated subgroup from the otherline;

2; In combination with? aoflong, dis tamce transmission. lines that'carry multiplex carrier telephonexsignals in the same frequency range and in: the same: direction of; transmission, saida lines lying adjacent"each other over a sub- .stantialqportion of their,lengtluirequency see lecti ve :means adapted to divert signals occu"- aying a fractional part of the total frequency range occupied by signals on one ofisa-id= lines, frequency selective means: adapted to divertfrom the otherloflsa-id ,lines'signals occupying the same fractional part of; theline frequency range; and, a; cross-talk: balancing coupling be-z tween said frequency selective: means. "1

, 3'; A ;long distance signaling: system comprising a pair of transmission lines in cross-talk producing" relation, means for transmitting through both of-said linesin the samesdirec= tion of transmission respective sets of multiplex-1 carrier telephone vstgnals occupying the same frequency rangeiinz ,both 'of 1 said lines; means for transmitting in the opposite: direction over both: of said lines respective sets of; multiplex carrier telephonelsignals occupying an other -common-,-frequency range, frequency se lective-greceiying means at a point common to v both of said lines'for separately receiving like directed sets ofsignals, means fortranslating said selected sets of signalsato-another common frequency range and means for crosstalk-bal ancing the translated sets. 1 a

t 4: In combination; a pairnof long-distance transmission linesth'atr; are in cross-talk pro ducing relation substantiallythroughout their length, meanstor transmitting through cboth of said lines-in the' samedirection o'fitran'smission-respectlve sets of multiplexoa-rrier tale:-

phone channels,;;said; sets occupyingthe same lineafrequency range; and comprisingamulti plicity of groups of channels, frequency selective means forselectively diverting from each of said lines and separating lfronr each other a plurality of said groups of channels, and means coupling the respective groups of channels selectedfrom the'one line into cross-talk balancing relationat line frequencies with: the respectively 'corre-' sponding groups selected irom the other line.

5. 'A combination in accordance with: claim 4 comprising in addition terminal circuit means adapted to: select and receive said groups of channels, means for demodulating. said groups of received channels to an intermediate carrier trequency range, and means for cross-talk balancing the separated, demodulated groups.

6. Ina system comprising a pair of multichannel carrier signaling circuits each carrying a multiplicity ofsignals in individual frequency bands. the method of reducing crosstalk between the circuits which comprises separating thesignals of both circuits into groups of signals occupying substantially contiguous frequency bands'and cross-talk balancing, the sep arated' groupsof the one circuit; against the respectively corresponding separated groups of the other circuit. 1 1 '1 t 1 7-. In a system comprising a pairof like-directed: multiplex carrier telephone transmission circuits, the method of reducing ,-intercircuit cross-talk which; comprises selectively; receiv ing constituent groups of carrier telephone signals frornyeachof, said circuits, translating said selected groups tothe "same frequency range and cross-talk balancing the translated groups from the one-"circuit against the translated groups from the other circuit 7 i j 1 JOHN T. DIXON.- 

